Whisk the cornstarch and remaining 1/4 c. milk together, then whisk it into the simmering mixture. Continue to simmer, whisking constantly, until sauce has thickened and is smooth, about 2 minutes.

Off the heat, gradually whisk in the chedar cheese until melted and smooth. Stir in the macaroni and let the macaroni and cheese sit off the heat until the sauce has thickened slightly, 2-5 minutes before serving.